1. Field of the Invention
The present invention relates to a group II-VI epitaxial layer grown on a silicon substrate. More particularly, the present invention relates to a (111) CdTe epitaxial layer grown on a (111) silicon substrate. HgCdTe is known as a sensitive material for infrared detection, and HgCdTe hetero-epitaxial growth on a silicon substrate is a key technology for development of monolithic infrared detectors. For this purpose, a CdTe buffer layer is first grown on a silicon substrate, and thereafter the HgCdTe layer is grown on the CdTe epitaxial layer.
2. Description of the Related Art
A conventional infrared imager of a hybrid type has a structure such that a plurality of infrared detector elements of HgCdTe are formed on a CdTe substrate and a signal processing arrangement, for example, a CCD is formed on a silicon substrate, and two substrates are integrated together using a plurality of bumps forming the infrared imager.
Response to requirements for an increased number of detector elements in order to improve resolution in image detection, an infrared imager of a monolithic type has been proposed. The monolithic infrared imager utilizes a CdTe buffer layer grown on a silicon substrate, and a HgCdTe layer is grown on the CdTe buffer layer. A signal processing arrangement is formed in the silicon substrate and a plurality of detector elements are formed in the HgCdTe layer. The monolithic infrared imager can cope with the increased number of detector elements, and enables simpler manufacturing processes and increases mechanical reliability.
The basic problem in production of the monolithtic infrared imager is to achieve hetero-epitaxial growth of CdTe on a silicon substrate. It is well known that crystal structure of silicon is of a diamond type and the crystal structure of CdTe is of a zinc-blende type. These two types has sufficient resemblance in lattice structures such that, if both two kinds of atoms of the zinc-blende type are replaced with silicon atoms, the crystal has the same structure as the diamond type of silicon.
In growing a CdTe epitaxial layer on a silicon substrate, the following methods are known, in which the grown CdTe layer has the same crystal orientation as that of silicon substrate. When a silicon substrate having a (100) plane is used, a CdTe epitaxial layer with a (100) plane is grown thereon. When a (111) silicon substrate is used, a (111) CdTe epitaxial layer is grown thereon. In both cases, the grown CdTe layer has the same in-plane direction as the crystal direction of the underlying silicon substrate. One aspect of these technologies is disclosed in "Growth of CdTe films on silicon by molecular beam epitaxy" by H. H. Stadelmaier, J. Appl. Phys. 54(7), July 1983, pp. 4238-4240.
The above methods include a basic problem that a lattice mismatch between silicon and CdTe can not be avoided. In the above case, the lattice mismatch runs to about 19%. It is known that GaAs has the same zinc-blende structure as CdTe and can be used as a substrate or a buffer layer on a silicon substrate. However, the lattice mismatch between GaAs and CdTe is reduced only to 14.6%.
The large lattice mismatch as described above causes crystalline defects such as misfit and dislocation. Other methods of growing a CdTe epitaxial layer with a different crystal orientation from that of a substrate have been tried; however, there still remains a lattice mismatch problem.